Wing-mounted engines on commercial aircraft can generate “shock-cell” noise that excites low frequency vibrational modes in the fuselage. The vibrational energy is transmitted through the fuselage by stiffeners and frames, and can cause substantial noise in the passenger cabin. This noise is often difficult to dampen, and is typically addressed by bonding relatively thick, metallic layers to portions of the stiffeners and/or frames to provide what is commonly referred to as constrained layer damping.
FIGS. 1A and 1B are end views of two constrained layer damping systems 110a and 110b, respectively, configured in accordance with the prior art. Referring first to FIG. 1A, the damping system 110a is attached to a longitudinal stiffener 102a which in turn is attached to a fuselage skin 108a. The damping system 110a includes a constraining layer 104a which is bonded to the stiffener 102a by an adhesive layer 106a. The constraining layer 104a is typically aluminum, and the adhesive layer 106a is typically a viscoelastic adhesive, such as one of the Scotch Damp Viscoelastic Adhesives products provided by the 3M™ Company under the ISD-112, ISD-113, or ISD-830 part numbers. Referring next to FIG. 1B, the prior art damping system 110b includes an angled constraining layer 104b attached to a stiffener 102b by means of an adhesive layer 106b. With the exception of the angle, the constraining layer 104b and the adhesive layer 106b can be similar in structure and function to their counterparts in FIG. 1A.
One downside of the prior art damping systems described above with reference to FIGS. 1A and 1B is that they can add significant weight to the base structure. For example, a typical installation of the configuration illustrated in FIG. 1A can weigh up to 0.9 pound per square foot of damping system. Another downside of these damping systems is that they can be difficult to manufacture and install.